| You want your investment to grow by a factor of 50 (that is 100,000/2000), so what you need to figure out is how many times do you need to multiply 1.04 by itself to reach 50? Stated as a math formula, solve for N where 1.04^N >= 50. Use logarithms and you can solve for N pretty quickly. If you don't know how to use lagarithms, you can estimate the answer using the "rule of 72" - which says that an investment at a rate if return of x% will double in about 72/x years. So here you can see that the investment will aproximately double in 72/4 = 18 years. You need the investment to grow by 50, so how many time does it need to double? Multiply that by 18 and you'll have an estimate of the correct answer. | 
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